fix(ui): prevent OpenAI API Host fallback when saving other fields

[bavadim]

Summary

Fixes a UI bug in OpenAI provider settings where saving changes to a non-host field can cause the effective API Host to fall back to default (https://api.openai.com) even when API Host was not edited.

The UI can still display the previously entered host text, so the regression is easy to miss.

Type of Change

• Feature
• Bug fix
• Refactor / Code quality
• Performance improvement
• Documentation
• Tests
• Security fix
• Build / Release
• Other (specify below)

AI Assistance

• This PR was created or reviewed with AI assistance

Testing

• Unit test (new):
  • ui/desktop/src/components/settings/providers/modal/subcomponents/handlers/DefaultSubmitHandler.test.ts
  • Covers OpenAI regression: update OPENAI_API_KEY without editing OPENAI_HOST, and assert host does not revert to default.
• Verification result:
  • Test fails on main (host falls back to default).
  • Test passes with this fix branch.

Related Issues

Relates to #7245
Discussion: #7245

Screenshots/Demos (for UX changes)

Before:
Changing another OpenAI field and saving could make effective API Host revert to default while UI still showed old host text.

After:
Saving changes to non-host fields preserves previously configured API Host.

[DOsinga]

thanks! somebody else also fixed this. since they have the DCO check, going to merge that one, but appreciate your efforts!